Optimizing N rate in wheat-maize rotation to match long-term and inter-seasonal N turnover for high yield and sustainability using STICS

Context: The winter wheat-summer maize rotation in China's Huang-Huai-Hai (HHH) farming region has been plagued by the long-standing problem of excessive nitrogen (N) application, driven by ambiguous N fertilizer recommendations, due to insufficient understanding of inter-seasonal N turnover and long-term N accumulation effects in the crop-soil system. Objective: Our aims were (1) to calibrate the STICS model for the wheat-maize rotation in the HHH region, (2) to elucidate the annual N turnover characteristics affected by seasonal differences in N use, mineralization and leaching, and (3) to determine optimal N rates considering long-term cumulative effects of N fertilizer management. Methods: Data from an 11-year field experiment were used to calibrate and evaluate the STICS model. Scenario analysis contrasting different N rates for wheat and maize, as well as their pairwise combinations, was conducted to identify optimal N ranges for each season, aiming for high yields and nitrogen-use efficiency (NUE) in concert with low N surplus. Results: STICS well captured the dynamics of shoot biomass (rRMSE: 20-22 %), N uptake (rRMSE: 22-28 %) and soil water content (rRMSE: 19-24 %) under different N treatments, respectively. STICS relatively well simulated crop yields under various N rates, with rRMSE of 9-13 % in the short term, and 13-15 % in the long term. Simulated N mineralization was higher in maize seasons than in wheat seasons due to the higher temperature and soil moisture, leading to a greater N surplus and increased leaching under the current management. Our simulations revealed optimal N rates of 180 kg N ha-1 for wheat and 164 kg N ha-1 for maize, which were 12 % and 38 % lower than contemporary N use in the region, achieving a long-term stable annual yield of 18.2 Mg ha-1, along with an annual NUE of 78 % and an N surplus of 80 kg N ha-1 per year. Conclusions: High residual soil N from the wheat season and strong N mineralization during the maize season suggest that less N fertilizer can be applied to maize without influencing yield level. Implications: A systematic perspective and consideration of long-term N turnover within crop rotations provided by crop models and field observations are crucial for improving N management in the wheat-maize rotation in the HHH region of China.